Precise sowing, either mechanically or by hand, has recently come into general use in order to save labor and stabilize growth in vegetable culture. Further, transplanting culture using a seedling tray, pot, etc., has become popular, too. With respect to the sowing into a seedling tray, it is said that accurate and precise sowing by machines or instruments is a necessary condition for the growing of good seedlings. Though the precise sowing is thoroughly possible to achieve mechanically in case of significantly large-sized, spherical seeds such as soybeans and red beans, it is very difficult to achieve mechanically in the case of odd-shaped seeds of lettuce, carrot, and Welsh onion, and in the case of small-grained seeds of cabbage and Chinese cabbage. Hand sowing of these seeds is inefficient, requiring much labor.
In order to overcome these disadvantages, it has become a common practice to coat seeds with a proper material to produce spherical grains larger than the seeds. The coating material for this purpose is a mixture of an inorganic matter such as diatomaceous earth, talc, clay, and calcium carbonate, and a binder such as starch, gelatin, and PVA (polyvinyl alcohol). Coated seeds thus produced have a problem. That is, when they are sown in a field where the groundwater level is high or the soil contains excess water due to rainfall after sowing, or in a soil having a high water content due to excessive flooding, the surroundings of the seeds are sealed with water, whereby supply of oxygen to the seeds becomes insufficient to cause poor germination. In order to overcome this disadvantage there was proposed the coating of seeds with a coating material containing a water repellent or waterproofing agent, as disclosed in Japanese Patent Application (OPI) No. 85908/1979 and Japanese Patent Publication No. 3469/1963. (The term "OPI" herein used means an "unexamined published Japanese patent application".)
With respect to these proposed methods, the present inventors carried out various testings. As the result, they found that even in these methods, if the soil in which the coated seeds are sown has a high water content due to a large quantity of rainfall or excessive flooding, poor germination still occurs, leading to insufficient results.
Germination of coated seeds generally takes place in the following manner: A coating layer of the coated seed sown in a soil absorbs water in the soil supplied due to rainfall or flooding and cracks into several portions, whereby a part of the naked seed is exposed from the coating layer. The seed receives oxygen mainly through these cracks and simultaneously absorbs water in the soil to germinate. At this time, if the water content is in excess, supply of oxygen to the seeds becomes insufficient, leading to poor germination. On the other hand, if the water content is too low, there is caused poor germination due to shortage of water. Accordingly, oxygen and water are indispensable for the germination of seeds, and if the quantity of water surrounding the seeds is controlled, stable germination can be achieved. One way of achieving this is to coat seeds with a coating material having mixed with and dispersed therein a water repellent or waterproofing agent, as described above. However, in this method, when there is a large quantity of rainfall or excessive flooding, excessive water surrounding the seeds which is not absorbed by soil infiltrates into the interface between the seed and the coating layer through the cracks of the coating layer, leading to a situation where the surroundings of the seeds are sealed with water to cause insufficient supply of oxygen to the seeds. Thus, the seeds become dead or damaged to cause poor germination.
As the result of extensive studies, the present inventors found that these problems can be solved by coated seeds obtained by covering the seed surface with a water repellent and then with a coating material. The present invention was completed based on this finding.